The present invention relates generally to nonvolatile memory devices and, more particularly, to electrically programmable read only memory (EPROM) devices having a uniform program characteristic and methods of programming the same.
Semiconductor memory devices are typically categorized as either random access memory (RAM) devices or read only memory (ROM) devices according to data volatility. The RAM devices lose their stored data when their power supplies are interrupted. In contrast, the ROM devices retain their stored data even when their power supplies are interrupted. The ROM devices may also be classified as programmable ROM (PROM) devices or mask ROM devices according to data input methods, that is, data program methods. The PROM devices may be fabricated and sold out without program and may be directly programmed by customers (i.e., users) after fabrication. The mask ROM devices may be programmed during fabrication using implantation masks manufactured based on data requested by users. The PROM devices may include one-time PROM (OTPROM) devices, erasable PROM (EPROM) devices and electrically erasable PROM (EEPROM) devices. Once the EPROM devices are programmed, the programmed data of the EPROM devices cannot be electrically changed but can be physically erased using, for example, ultraviolet (UV) rays.
N-channel transistors or P-channel transistors may be used as cell transistors of the EPROM devices. If P-channel transistors are used as the cell transistors of the EPROM devices, the P-channel cell transistors may have a turn-off status as their initial status and may have a turn-on status as a programmed status. Thus, a read operation of the EPROM devices may be performed by sensing cell currents that flow through the P-channel cell transistors.